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The world will have to reduce its use of fossil fuels to less than a quarter of the proven reserves by 2050 if it wants to stay within 'safe' climate change limits, according to a new report.

The paper, published in the journal Nature, implies a revolution in energy use is needed to achieve the aim of limiting warming to less than 2°C above pre-industrial levels.

To achieve the objective means that only 1000 billion tonnes of carbon dioxide (CO2) can be emitted between 2000 and 2050, it says.

By comparison, the world has emitted a third of that amount in just nine years.

"If we continue burning fossil fuels as we do, we will have exhausted the carbon budget in merely 20 years, and global warming will go well beyond two degrees," says the study's lead author, Dr Malte Meinshausen of the Potsdam Institute for Climate Impact Research in Berlin.

Fossil fuels, which include coal, gas and oil, provide the backbone of the world's energy supplies.

But they are also a source of greenhouse gases, seen by many as warming earth's atmosphere and driving changes to weather patterns.

Boosting efficient use of these fuels or switching to cleaner alternatives carries an economic cost, and this is the biggest stumbling block in efforts to reduce CO2 output.

Urgent action

Meinshausen says the change should not be delayed and cautioned that even with a 2°C warming, there would still be unprecedented risk.

"Only a fast switch away from fossil fuels will give us a reasonable chance to avoid considerable warming," he says.

"We shouldn't forget that a 2°C global mean warming would take us far beyond the natural temperature variations that life on earth has experienced since we humans have been around."

The study says world emissions of greenhouse gases have to be cut by more than 50% by 2050 compared to 1990 levels if the risk of busting the 2°C ceiling is to be limited to 25%. In addition, reductions would have to be made from 2020.

The Group of Eight (G8) countries have pledged an emissions reduction of at least 50% by 2050.

But they have not identified a benchmark year against which this should be measured, nor set an intermediate date by which emissions cuts should start.